I also think that Blackbeard's relationship with Izzy was a little codependent. That's why he's manipulated in the end. That's why - when their last interaction was being betrayed by Izzy - he returns and asks for his tea like nothing happened. Izzy gives him security.
And I think he's only able to let Izzy leave him after he has the moment in the bathtub with Stede. Where Stede makes him feel safe and like he'll take care of him.
So he's finally strong enough to let Izzy go after the duel, because he has Stede.
But the next episode, the treasure hunt, we see him grumpy. He's sad about Izzy. And he doesn't trust that he can just stay with Stede - most importantly he doesn't trust that Stede will want him to stay.
He only decides to commit after Lucius reassures him of Stedes feelings.
But he's still waiting for Stede to realize that he's a bad person, and maybe not worthy. Which is why he leaves with Jack. He tells stede "this is who I am", and he feels stede would always reject him for this part of his personality. #OurFlagMeansDeath
I think he feels okay about the act of grace, not just because he had just imagined the horror of Stede's death and would do anything to prevent it. And now he knows he wants nothing more than to stay with him.
But also because he thinks Stede doesn't care for his pirate side.
So he happily gives up that part of himself - and is happy to do so because he's been bored of it for ages.
In contrast, Stede actually sees Blackbeard for his whole self. That's why he recognizes that Calico Jack isn't good for him, but also why he recognizes that Ed giving it all up at the privateering academy is also not ideal. #OurFlagMeansDeath